Many biochemical tests formerly performed in the laboratory using advanced equipment and skilled technicians can today be performed by a physician, a nurse or even the patient himself/herself, using small, often disposable devices. This is one result of a better understanding of biochemistry and medicine, as well as the ongoing miniaturization of both mechanics and electronics, taking place over the recent decades.
Such tests can be divided into two groups: “one-step tests” where a reaction takes place on a substrate after the addition of sample, and the result is detected as a change of one or more properties of said substrate; and “two-step tests”, where the sample is followed by the addition of a detection conjugate, leading to a specific reaction resulting in a detectable signal.
In most assays, the detection conjugate and possible other reagents are pre-dispensed or integrated in the device, setting aside the need for separate addition of reagents by the user.
The most common type of disposable assay device consists of a zone or area for receiving the sample, a reaction zone, and optionally a transport or incubation zone connecting the receiving and reaction zone, respectively. These assay devices are known as immunochromatography assay devices or simply referred to as strip tests. They employ a porous material, such as nitrocellulose, defining a fluid passage capable of supporting capillary flow. The sample-receiving zone frequently consists of a more porous material, capable of absorbing the sample, and, when the separation of blood cells is desired, effective to trap the red blood cells. Examples of such materials are fibrous materials, such as paper, fleece, gel or tissue, comprised e.g. of cellulose, nitrocellulose, wool, glass fibre, asbestos, synthetic fibres, polymers, etc. or mixtures of the same. The transport or incubation zone commonly consists of the same or similar materials, often with different porosity than that of the sample-receiving zone. Likewise, the reaction zone, which may be integrated with the incubation zone, or constituting the most distal part thereof, commonly consists of similar, absorbing fibrous materials, such as nitrocellulose, or any of the above listed materials.
Nitrocellulose materials are also frequently used as the matrix constituting the transport or reaction zone, or connecting the receiving zone and the reaction zone. A significant disadvantage with nitrocellulose is its high non-specific binding of proteins and other bio-molecules. Present test strips however often handle a surplus of sample, reducing the influence of this binding. Another disadvantage of nitrocellulose is its variations with regard to both chemical and physical quality. It is in any case desirable to minimize the sample volume, in line with the tendency to miniaturize the entire test, including minimizing the amounts of reagents, without compromising accuracy and reliability.
In an assay device or strip test, the porous material/-s is/are assembled on a carrier, such as a strip of thermoplastic material, paper, cardboard or the like. Further, a cover can be provided, said cover having at least one aperture for receiving the sample, and an aperture or a transparent area for reading the result of the assay.
Frequently this cover is simultaneously a housing or case, enclosing the porous material, providing stability and structural protection. Examples of such constructions include published U.S. patent application Ser. No. 10/794,516, published as 2004171174, international publication number WO2004/038414, or U.S. Pat. No. 6,846,453.
U.S. Pat. No. 6,312,888 discloses an assay device comprising several layers for the analysis of an analyte in a biological sample.
From WO2005/089082 there is known a device for handling liquid samples, comprising an area having projections substantially vertical to its surface, whereby the projections create a capillary force. In such assay devices there arise new problems compared to earlier assay devices without projections creating a capillary force.
Problems in the state of the art regarding assay devices include how to improve the sample addition, how to improve the hematocrit tolerance for blood samples, and how to protect the assay device from damage from a pipette, when a pipette is used to add a liquid to said assay device.